1. Field of the Invention
This invention relates to solid, particulate fertilizers, and more particularly to solid, particulate, nitrogen-base fertilizers containing micronutrients and to their method of manufacture.
2. Description of the Prior Art
For many decades, urea has been used in large quantities as a fertilizer material. Its relative popularity over other forms of plant-available nitrogen results from the fact that, in many localities, it is the least expensive source of nitrogen. In addition, urea has the highest nitrogen content of any solid fertilizer material, so that smaller quantities can be applied to a given plot of land to achieve a desired level of available nitrogen.
Urea for fertilizer use is ordinarily marketed in the form of prills. Prilling produced a free flowing, non-dusting material which is easily handled and applied to the soil with spreaders normally used on farms. Upon prolonged storage, however, the relatively soft, hygroscopic urea prills tend to agglomerate and cake into large masses. Due to this tendency, the urea is frequently packaged in water-repellent, multi-layered bags for protection from atmospheric moisture, but caking is still noted after the bags have remained stacked during the usual warehouse storage.
Various procedures have been devised for decreasing the caking properties of urea prills, including the application of coating agents. The usual coating process has consisted of dusting the prills or otherwise admixing the prills with coating powder. Powders used have included various silica compositions and silicates, diatomaceous earth, clays and several inorganic minerals. Such coatings, however, have not proven to be entirely satisfactory due to the rather loose adhesion obtained. During handling, a significant portion of the coating becomes dislodged, leaving surfaces of the prill unprotected against caking.
To overcome this deficiency, non-powder coatings such as molten sulfur, oils, urea-formaldehyde mixtures, or molten organic compositions (e.g., waxes, bitumen and the like) have been applied to urea. The result has generally been a significant improvement in anti-caking properties and, depending upon the specific coating used, the attainment of slow-release characteristics for the urea, but accompanied by a sharp increase in product price. Such coated products have therefore been of practical use mostly for crops which have a high profit margin or in home gardening uses.
One further approach has been that of "encapsulating" the urea inside an inorganic shell, such as in U.S. Published application No. B 141,968 to Kawar. Particles are coated with a metal oxide or hydroxide (e.g., calcium oxide or calcium hydroxide) and the coated material is exposed to carbon dioxide, forming a low-porosity coating of metal carbonate.
Other workers have devised systems wherein coatings are applied which are capable of reaction with the urea particle, which reaction will form a relatively more impervious or lesser caking composition at the surface of the prill. Much of this effort has been with the hydrated metal sulfates, as in U.S. Pat. No. 2,074,880 to Whittaker et al. (gypsum--CaSO.sub.4 .multidot.2H.sub.2 O) and also in U.S. Pat. No. 3,867,124 to Church (FeSO.sub.4 .multidot.7H.sub.2 O, MgSO.sub.4 .multidot.7H.sub.2 O, CuSO.sub.4 .multidot.5H.sub.2 O), which describes the coating of ammonium nitrate particles with the reaction product of urea and the metal sulfate hydrate. According to these patents, the water of crystallization in the metal sulfate is replaced by urea, yielding a liquid product which forms the coating. Drying is then needed to remove the displaced water molecules.
Another method for coating fertilizer particles is that of Wadsted in U.S. Pat. No. 3,684,476, whereby dolomite is selectively calcined to form a material having from one to five moles of calcium carbonate per mole of magnesium oxide. Urea is reacted with this material, in the solid phase, to give magnesium dicarbamide and some magnesium carbamide, which coats a fertilizer particle. Calcium carbonate attaches to the reaction product, forming a protective layer.
As a labor-saving consideration, it is frequently desirable to simultaneously apply major nutrients, such as nitrogen, and micronutrients, such as trace metals, to crop-growing land. Tucker et al., in U.S. Pat. Nos. 3,941,578 and 3,981,713 describe two methods for combining urea with zinc oxide. The former method consists of simply coating urea particles with very finely divided zinc oxide powder, reportedly suffering to a lesser degree from the previously noted adhesion problems, which result in caking of the urea and separation of the ingredients. The latter method involves dissolving zinc oxide into molten urea, giving a good dispersion of the micro-nutrient but not significantly affecting the caking tendencies of urea.
The previously noted processes for reacting urea with hydrated metal sulfates also may be viewed as describing the incorporation of micronutrients into fertilizers, insofar as iron and copper are useful in promoting the growth of plants.
A need remains, however, for coated, non-caking urea fertilizer particles which are prepared by bonding micronutrient oxides to the particles, using a method which does not involve a lengthy drying step.
Accordingly, it is an object of the present invention to provide urea particles which are protected against caking by a surface layer composed of a relatively hard, micronutrient-containing product of the reaction between urea and one or more metal oxides.
A further object is to provide solid fertilizer particles having a core of materials other than urea, upon which a surface layer is composed of a micronutrient-containing product of the reaction between urea and one or more metal oxides.
It is also an object of the invention to prepare the aforementioned particles by an uncomplicated method which can utilize simple equipment common to the fertilizer industry.
An additional object is the preparation of coated, non-caking urea particles by a method which does not require a drying step for the product.
These and other objects will more clearly appear from consideration of the following description and examples.